The use of solid rods and/or hollow pipes as grinding media has been common practice in varied industries for many years. For example, they have been used as grinding media in mills having enclosed grinding chambers which rotate about their longitudinal axes, generally at speeds less than critical speed. Typical units are illustrated in FIGS. 1 and 2.
The rods and pipes, when used in mills with cylindrical grinding chambers, form a kidney-shaped tumbling mass adjacent the up running side of the rotating cylinder when viewed from either end of the cylinder and, as the cylinder turns, the rods and pipes migrate and move rotationally through the mass and about their own axes, crushing and grinding the material to be reduced between the rolling or moving surfaces of the rods and pipes. These mills generally include means for retaining the rod and/or pipe axes substantially parallel to the grinding chamber rotational axis. This can for example be accomplished in a circular cylindrical grinding chamber by providing it with a length in excess of its diameter. For instance, a length to diameter ratio of at least about 1.2 or preferably at least about 1.3 will be sufficient to prevent the rods and pipes from standing on end and/or tangling with one another in most instances.
While pipes and rods both produce a limited number of fines in the ground product, rods generally produce a finer particle size distribution than pipes. Because pipes are less dense than rods, a pipe mill is typically of larger cross-section and length than a rod mill which draws the same amount of power. In the pipe mill the throughput is also larger, because the mill cross-section that the material traverses is larger than that in the above-mentioned rod mill. Continuing our comparison of these two mills of equal power draw, the pipe mill tends to produce a larger weight of product per unit time, but does less work on the material per unit of material weight, resulting in a larger particle size product than that produced by the rod mill.
Characteristics of the material being ground also play a role in how fine a product is produced, which role is at least in part independent of the density of the grinding medium and hence of the power to achieve the results. This is also reflected in the size distribution of the ground product. It is for example well known that when a material with flaws or grain structure boundaries is subjected to a compression or impact load, a large particle of the material will break more readily than a smaller particle.
It is believed that there is a need for a mill which can provide increased throughput capacity with smaller cross-section while minimizing excessive generation of fine particles. The primary object of this invention is to fulfill this need.